Vapour layer, Dormer knee walls and increasing knee-wall insulation

ravendude

Hi there,

(Posted also in DIY, but I'm not sure if it is more appropriate here)
I have knee walls on my dormer. They are chalk board backed with fibreglass roll (70mm approx).
The chalk board has a fairly thick foil like back to it and seems to be taped with at the joints again with foil tape. Would this act as a vapour control layer (to some extent)?
Surely vapour cannot permeate this foil backing from within the dormer rooms?

The reason I ask is that I recently added to the knee wall insulation by stapling 200mm Knauf space blankets (designed for attic ceiling level) to the backs of the knee wall, over the existing fibreglass roll. This is the only way I can think of to improve matters without major rework. It seems to be making quite a noticeable difference already, though the real test will come in a few months time.

Obviously, - condensation is the concern...

The space blanket is mineral wool, encased in a perforated foil plastic membrane covering. The Knauf product datasheet ( http://www.space-insulation.com/wp-c..._Datasheet.pdf ) states that the cover adds "negligable resistance" to vapour, - which I take to mean it is basically breathable. I intend to monitor carefully for condensation over the forthcoming weeks/months/years, - I have seen no sign (yet) in the last few weeks anyway. Is there any danger from something I can't see (eg. dry rot?) ?

I'm thinking if the knee wall boards are backed with some kind of vapour control (the foil I refer to above?), - it might be fine anyway?

Thanks!

BryanF

ravendude wrote: »

Hi there,

(Posted also in DIY, but I'm not sure if it is more appropriate here)
I have knee walls on my dormer. They are chalk board backed with fibreglass roll (70mm approx).
The chalk board has a fairly thick foil like back to it and seems to be taped with at the joints again with foil tape. Would this act as a vapour control layer (to some extent)?
Surely vapour cannot permeate this foil backing from within the dormer rooms?

The reason I ask is that I recently added to the knee wall insulation by stapling 200mm Knauf space blankets (designed for attic ceiling level) to the backs of the knee wall, over the existing fibreglass roll. This is the only way I can think of to improve matters without major rework. It seems to be making quite a noticeable difference already, though the real test will come in a few months time.

Obviously, - condensation is the concern...

The space blanket is mineral wool, encased in a perforated foil plastic membrane covering. The Knauf product datasheet ( http://www.space-insulation.com/wp-c..._Datasheet.pdf ) states that the cover adds "negligable resistance" to vapour, - which I take to mean it is basically breathable. I intend to monitor carefully for condensation over the forthcoming weeks/months/years, - I have seen no sign (yet) in the last few weeks anyway. Is there any danger from something I can't see (eg. dry rot?) ?

I'm thinking if the knee wall boards are backed with some kind of vapour control (the foil I refer to above?), - it might be fine anyway?

Thanks!

id have a vapour barrier/air-tightness layer behind the plasterboard but inside (warm side of) the insulation. i appreacite your not keen on retro-fitting this sort of detail. so id suggest at the very least where moisture is being generated ie in Wc's, kitchen, drying clothes, sleeping etc - that proper ventilation & minimum heat is provided. if the knee-high space is properly vented at the eaves/ridge etc then you should be ok.

ravendude

The eaves have the usual soffit vents so it should be OK hopefully.

The foil at the back of the plasterboard. - what is the purpose of this, is it not some kind of vapour barrier?

BryanF

ravendude wrote: »

some kind of vapour barrier?

yes 'some kind of' being the key words - as its rarely continuous - you have sockets, lights, junctions, penetrations, floors etc and they let moisture form your heated home into a cold surface/space to condense.

ravendude

BryanF wrote: »

yes 'some kind of' being the key words - as its rarely continuous - you have sockets, lights, junctions, penetrations, floors etc and they let moisture form your heated home into a cold surface/space to condense.

Ok, makes sense, - thanks for that. Hopefully the space has adequate ventilation anyway, and the insulation is breathable enough (should be by the looks of it). The eaves have the soffit vents and there is a gap above the knee walls (on the cold side) for the slopes to breath. I'll keep an eye on it anyway and if there are signs of moisture that are not drying, it will come out.
Thanks again.

ravendude

On a related note, - I noticed that the dormer conversion isn't blocked off at the ceiling joists under the knee wall? Should they be blocked with plywood, dense insulation or a membrane etc. to prevent cold air from entering the conversion under the floorboards? - or should the insulation suffice?

The joists of course have insulation in between and on top (fibreglass and mineral wool over that) to just under where they meet the knee wall. Beefing up the insulation on the knee wall has the additional benefit of increasing where it meets the ceiling joists also, so less cold air going under the floowboards. Not sure if this is enough though. I was thinking of blocking the ceiling joists (uit would need to be a DIY job for now) with that radiant barrier foil wrap stuff just to help block cold air from going under the floor boards.

BryanF

ravendude wrote: »

On a related note, - I noticed that the dormer conversion isn't blocked off at the ceiling joists under the knee wall? Should they be blocked with plywood, dense insulation or a membrane etc. to prevent cold air from entering the conversion under the floorboards? - or should the insulation suffice?

ideally the habitable space on the warm side of the insualtion is completely sealed from the external, generally by means of an air-tightness layer - there have been a few great threads which i dont have to hand on this topic - but ultimately its a tough retro-fit

The joists of course have insulation in between and on top (fibreglass and mineral wool over that) to just under where they meet the knee wall. Beefing up the insulation on the knee wall has the additional benefit of increasing where it meets the ceiling joists also, so less cold air going under the floowboards.

insulation is not air-tight

Not sure if this is enough though. I was thinking of blocking the ceiling joists (uit would need to be a DIY job for now) with that radiant barrier foil wrap stuff just to help block cold air from going under the floor boards.

should be done on warm side of the insulation only

careful not to block attic (cold space) ventilation